Method and apparatus for making load cells less sensitive to off-center load applications

ABSTRACT

A measuring pick-up or load cell shall be made less sensitive against off-center load applications, if necessary. At least it must be made certain that the off-center load sensitivity is within a permissible tolerance range. For this purpose the sensor body (1A, 1&#39;, SB) of the detector (1) to which there is applied at least one pick-up element or strain gage (5), is held for each load application in a different fixed position in a mounting (2) in such a way that, spaced from the clamping position a test load can be applied in a direction deviating from the direction and/or position of the axis of symmetry (CA, AS) of the sensor body (1A, 1&#39;, SB). The test loads may be uniform or they may differ for one or more load application points. A respective output signal is measured and correlated to the applied test load. At least one additional test load of the same size is applied in a different load application direction, and the respective output signal is again measured and correlated to the second test load. Then, the output signals are evaluated with reference to a tolerance range (TR) stored in a memory of a control unit (11) to determine whether a tuning of the sensor body is necessary to make the pick-up less sensitive. If such tuning is necessary, at least one tuning is performed. The tuning may be a mechanical material removal from the sensor body and/or as an electrical tuning by resistor adjustments.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation-In-Part application of ourcommonly assigned application U.S. Ser. No. 08/433,617, filed on May 3,1995, now U.S. Pat. No. 5,679,822, issued on Oct. 21, 1997.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for making measuringpick-ups or load cells less sensitive to variations in the loadapplication, such as variations in the direction of the loadapplication, variations in the size and type of load and variations inthe point of load application. All types of variations are referred toherein as off-center load applications or as load applicationconditions. A tuning procedure achieves a reduced sensitivity tooff-center load applications.

BACKGROUND INFORMATION

Measuring pick-ups or load cells such as force pick-ups or load cellshaving a sensor body to which at least one strain gage is applied, aregenerally sensitive to variations in the load application conditions.Thus, the measuring characteristics of such pick-ups may change when theload application conditions change. For example, different measuringresults may be obtained for the same applied load if even minor angulardeviations occur in the direction of the load application such as anangular deviation from a vertical load application direction or in thepoint of load application. Other causes such as non-uniformities andnon-symmetries in the construction of the sensor body and/or in thestrain gage element or elements applied to the sensor body providefurther sources for variations in the measuring characteristic of suchpick-ups. These other causes are due to manufacturing tolerances whichcannot be avoided for economic reasons.

In order to assure exact measuring results that are consistentlyrepeatable, it is necessary to take steps that eliminate the possiblyadverse influences of load application conditions that may vary due tothe above mentioned causes. However, conventionally these stepsgenerally have to be taken after the installation of the measuringpick-up for example in a scale in the manufacturing plant. Suchconventionally necessary steps are bothersome because not only are theytime consuming, they also require a complex correction procedure. Itwould be substantially easier to desensitize or tune pick-ups prior totheir installation to make them less sensitive to varying loadapplication conditions.

European Patent Publication EP-A 0,089,209 (Griffen et al.), publishedon Sep. 21, 1983, calls for the application of testing loads to a scaleafter the load receiving scale platform has been rigidly connected tothe measuring pick-up and after the measuring pick-up has been rigidlyconnected with the scale base, whereby these testing loads are to beapplied prior to any compensating steps, for example, in the form ofmaterial removals or resistor adaptations in the strain gages.

Japanese Patent Publication 62-107033 (Tanaka), published on Nov. 10,1988, discloses a device with additional pick-up elements or straingages positioned so that loads applied to the measuring pick-up causedby undesired moments or lateral forces, are picked-up and electronicallycorrected, whereby the measured value is corrected and a corrected loadvalue is displayed. Even such an arrangement with additional pick-upelements or strain gages requires additional steps to be taken in theplant of the scale manufacturer.

Other attempts to minimize the effects of off-center load applicationshave been made, for example, by decoupling of lateral forces with theaid of roller bearings. These attempts also have their drawbacks, sincesuch structural features with roller bearings are very expensive.Similar considerations apply to constructions in which the measuring orsensor body has a complicated and hence expensive geometry or where amultitude of pick-up elements, namely strain gages, are applied to thesensor body. All these conventional attempts aim at eliminating theeffects of non-uniformities or non-symmetries in the sensor bodies or ofangular or positional variations in the load applications or variationsin the application of the strain gages to the sensor bodies.Incidentally, in this context, the term "measuring body" and the term"sensor body" are used as synonyms.

OBJECTS OF THE INVENTION

In view of the above it is the aim of the invention to achieve thefollowing objects singly or in combination:

to provide a simple, yet effective procedure and apparatus for makingmeasuring pick-ups or load cells less sensitive to the effects ofchanges in the load application and in the type of load applied andthereby also to effects of manufacturing tolerances that must beaccepted to avoid unnecessarily expensive manufacturing procedures;

to provide a tuning procedure that can be performed in the manufacturingplant prior to installation of the pick-ups to make the pick-up alsoless sensitive to effects other than the intended load effect;

to assure that all sensors of the same type or model have a uniformresponse characteristic in the intended load range;

to avoid the need for complicated and hence expensive sensor bodygeometries, and to also avoid the use of a multitude of sensor elementssuch as strain gages;

to avoid expensive tuning procedures after installation of such pick-upsinto a scale or the like;

to provide a program controlled tuning of load cells that takes intoaccount the application of different loads to the load cell for thetuning;

to perform tuning operations with the application of loads that differ,vary, fluctuate or oscillate during the load application; and

to provide a load cell tuning apparatus that includes a signalprocessing circuit for standardizing signals measured in response to theapplication of different loads, varying loads, fluctuating loads andeven oscillating loads to provide the results of a uniform loadapplication for the tuning operation.

SUMMARY OF THE INVENTION

According to the invention there is provided a load measuring pick-up orload cell having a sensor body with a symmetry axis, which is made lesssensitive to off-center load applications and to load applicationconditions and thereby also to manufacturing tolerances, by thefollowing steps:

(a) securing at least one pick-up element or strain gage to said sensorbody and holding the sensor body by at least one mounting at a mountingpoint,

(b) applying a first load e.g. an off-center load in a first directionrelative to said symmetry axis to said sensor body to produce a firstoutput signal,

(c) measuring said first output signal and correlating said first outputsignal to said first load,

(d) applying at least one second different load to said sensor body in asecond direction different from said first load application direction,

(e) measuring a second output signal and correlating said second outputsignal to said second different load,

(f) ascertaining from said first and second output signals whether anoff-center load sensitivity of said pick-up or load cell is within apermissible tolerance range, and if necessary as a result of saidascertaining

(g) performing at least one mechanical and/or electrical tuning of saidpick-up or load cell for making it less sensitive to such conditions.

According to the invention, there is also provided an apparatus formaking a load measuring pick-up or load cell having a sensor body (1A,SB) with a symmetry axis and at least one strain gage (5) applied tosaid body, less sensitive to off-center load applications and varyingload conditions and thereby also to manufacturing tolerances, comprisinga load application device (3, 3'), at least one mounting (2, 2') forsaid sensor body, a rotating device (M) for providing relative rotationbetween said sensor body (1A, SB) and said load application device (3,3'), a control unit (11) having at least one input and a number ofcontrol outputs, a rotation sensor (10) connected to said at least oneinput of said control unit (11) to provide an input signal representinga relative rotational position (I, II, III, IV) between said sensor bodyand said load application device to said control unit (11), a signalamplifier (13) connected to said strain gage (5) to receive outputsignals from said strain gage (5), a comparator (12) connected to anoutput of said amplifier (13) and to a first control output of saidcontrol unit (11) providing rated tolerance values for comparing withsaid strain gage output signals, a tuning device (15), said comparator(12) having a first output connected with an input of said tuning deviceor tool (15), a display unit (14) connected to a second output of saidcomparator (12), for enabling said tuning device (15) to perform atuning operation in response to a signal from said control unit (11) andin response to said comparator, and wherein said tuning device (15) hasan output connected to a further input (14A) of said display unit (14),and circuit components (20, 21, and 22) for equalizing or normalizingmeasured signals resulting from the application of at least onedifferent testing load in at least one different load application point(I, II, III, IV). Preferably, the normalization takes place withreference to the smallest applied load.

The foregoing teachings of the invention avoid the conventional tuningof measuring pick-ups after these pick-ups have been installed in ascale or the like. The invention avoids complicated sensor bodyconfigurations or structures. The simple and hence inexpensive tuning ofthe sensors or pick-ups in the manufacturing plant makes the presentsensors substantially non-sensitive to influences of lateral orcross-forces and thus against variations in the load applicationdirection and position as well as in the size of the load applied forthe purpose of ascertaining whether and to what extent a tuningoperation is required. This advantage makes sure that all pick-ups ofthe same model or type can be tuned in the plant so that each will havethe same response characteristic in the intended load range independentof off-center load applications, whereby the number of rejects isminimized because any required corrective measures can be simplyperformed when the pick-ups are not yet installed in their final workingposition in a scale or the like.

Another advantage of the invention is seen in that very simple featuresor steps permit the reduction of the so-called lateral sensitivity asmuch as necessary. The invention makes it possible for the first time todisregard complicated sensor body geometries while simultaneouslyavoiding the use of a multitude of strain gage elements applied toconventional sensor bodies. Yet, the invention achieves pick-ups thatcan be precisely manufactured for use for various purposes, for example,for mounting by a so-called pendulum mount or rocker pin load cell.According to the invention, precise measurements can be made even if theforce application conditions should vary during actual measurements.This feature is especially advantageous for scales in which the pick-upsare no longer rigidly connected with a load introduction element on theone hand and with a bearing support in the scale base on the other hand.

A further advantage of the invention is seen in that the degree ofdesensitizing is fully within the control of the operator by repeating achecking and tuning operation if that should be necessary to obtain acertain desensitation. Further, a clear evaluation result is assured byapplying the test load in two different load planes of the sensor body.Preferably, these load planes are positioned at right-angles relative toeach other. A sensor body having a rectangular, especially squarecross-section through that portion of the sensor body which forms thestrain gage application surfaces, is particularly suitable for thepresent purposes. A sensor body with a square cross-section is easy tomake.

By making sure that sequentially applied loads or forces to the sameload plane of the sensor body have oppositely effective directions, itbecomes easy to control the testing by checking the plus or minus signand the size of the resulting output signals. If the measured outputsignals produced by sequentially applied testing loads have oppositeplus or minus signs and substantially the same size, the measuringbecomes precisely repeatable and the tuning by material removal and/orby resistor tuning becomes easily possible.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now bedescribed, by way of example, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a side view, partially in section, of a testing apparatusaccording to the invention holding a sensor body of a pick-up andincluding a load applicator for applying a testing load to the sensorbody;

FIG. 2 is a view in the direction of the arrow AII with the loadapplicator omitted;

FIG. 3 shows a second embodiment of an apparatus according to theinvention similar to that of FIG. 1, however, with a modified loadapplicator;

FIG. 4 is a block circuit diagram of an apparatus for performing thepresent testing and tuning;

FIG. 5 shows four different measured output signals I, II, III, IVcaused by respective load applications at different angular positions ofthe sensor body rotated in 90° steps;

FIG. 6 shows a sectional view through a modified pick-up according tothe invention having a hollow cylindrical cup-shaped sensor body with acentrally positioned load introduction member;

FIG. 7 shows a view into the hollow, cup-shaped sensor body of FIG. 6 toillustrate the arrangement of strain gages;

FIG. 8 illustrates the measured output signals as a function of 90°rotation steps of the sensor body with an allocation of the respectivebridge circuit branches shown in FIGS. 11 and 12 to the correspondingrotational quadrant;

FIG. 9 is a view similar to that of FIG. 6, but illustrating the use ofa twin set of four strain gages each;

FIG. 10 is a view similar to that of FIG. 7 to show the positioning ofthe two sets of strain gages inside the cup-shaped, hollow sensor body;

FIG. 11 shows a bridge circuit diagram for the arrangement of straingages as shown in FIGS. 6 and 7 for an electrical tuning;

FIG. 12 shows a bridge circuit for the strain gage arrangement shown inFIGS. 9 and 10 for an electrical tuning;

FIGS. 13 to 16 illustrate several possibilities for the application of atesting load to the sensor body of a pick-up;

FIG. 17 is a modified tuning apparatus for tuning load cells whileapplying different test loads; and

FIG. 18 is a diagram similar to that of FIG. 5, but showing measurementsmade in an apparatus as shown in FIG. 17.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

Referring to FIGS. 1 and 2, the present testing and tuning method forpick-ups or load cells can be performed by a very simple apparatus inwhich a pick-up or load cell 1 is mounted in a chuck 2 for rotation withthe chuck 2 which is rotatably mounted in a machine frame MF. A motor Msteps the chuck 2, preferably in 90° steps for rotation about a centralsymmetry axis CA of the sensor body 1A of the pick-up 1. The sensor body1A has a clamping end 1B held in the chuck 2 and a free load applicationend 1C reaching into a load applicator 3 for applying a testing force Fto the free end 1C of the sensor body 1A when the body 1A is held in astationary position with one of its longitudinal edges facing up. Theload applicator 3 is, for example, moved by a piston cylinder device PCfor applying a defined load. It is possible to keep the sensor body 1Astationary altogether and stepping instead the applicator 3 with itsdrive PC around the axis CA in 90° steps.

As best seen in FIG. 2, in which the load applicator 3 is not visible,the sensor body 1A has a square cross-sectional configuration betweenits cylindrical ends 1B and 1C. The square configuration has four flatsurfaces 4A, 4B, 4C and 4D. One strain gage 5 is applied to at least oneof these flat surfaces 4A to 4D. In this example embodiment four straingages 5 are applied one to each surface.

As seen in FIG. 1, the load applicator 3 has a through-hole 3A which hasa larger diameter than the diameter of the cylindrical free end 1C ofthe sensor body 1A so that the load applicator 3 is connected to thesensor body 1A in the manner of a pendulum mounting.

The chuck 2 may, for example, be a conventional three jaw chuck which isoperated hydraulically or pneumatically. Any rotations of the chuck 2may even be performed manually by the operator or, as mentioned, by astepping motor M of conventional construction. The operation of thestepping motor and the respective load application by the applicator 3is preferably coordinated and controlled by a program stored in a memory11B of a control unit 11 shown in FIG. 4. The chuck 2 is lockableagainst further rotation in each of its rotated positions by aconventional locking device not shown. The locking device may beoperated manually or automatically also in response to the program. Amarker 10 is applied to the chuck 2 or to the sensor body 1A to indicatethe particular rotational position. One such marker will be provided foreach of the four 90° spaced positions.

A bending load is applied when the sensor body 1A is in the respectiverotationally stepped position by lowering the applicator 3, for examplewith the piston cylinder device PC. Four rotational positions I, II,III, and IV are shown in FIG. 2. These rotational positions are spacedfrom one another by 90° and uniform or differing loading forces F orF_(I), F_(II), F_(III), F_(IV) are applied to the sensor body 1A in eachof these four load application positions or points I to IV for a testingsequence.

As seen in FIG. 2, in each of the load application positions I to IV,the flat planes 4A, 4B, 4C, and 4D are inclined by an angle of 45° tothe horizontal plane HP and similarly by an angle of 45° to the verticalplane VP. Each of these planes 4A to 4D carries, as mentioned, forexample one strain gage 5. The strain gages of all four sides arepreferably interconnected to form a conventional bridge circuit.

Referring to FIG. 4, the bridge circuit formed by the strain gages 5 isconnected with its output to an amplifier 13 which in turn is connectedwith its output to one input 12A of a comparator 12. The comparator 12has a second input 12B connected to the above-mentioned control unit 11which has an input 11A connected to the output of an angular positionsensor 10A that senses the marker 10. A tuning device 15 including atool, such as a grinder shown in FIG. 1, is connected with one of itsinputs 15A to the output of the comparator and with its other input 15Bto the output of the control unit 11. A display 14 is connected with itsinput 14A to the output of the tuning device 15 and with its input 14Bto a further output of the comparator 12. The control unit 11 isessentially a central processing unit that provides reference values orsignals to the comparator 12. The reference values are stored in amemory 11B of the central control unit 11. The reference values aresupplied from the control unit 11 to the input 12B of the compara-tor 12in response to input signals received from the sensor 10A. The referencevalues establish the tolerance range in the comparator 12. A testingsequence program is stored in the memory 11B.

In operation, the sensor body 1A is turned into a starting position, forexample I, and locked. Then the load applicator 3 is activated bypressurizing PC. Following temperature equalization, the output signalfrom each strain gage signal is sensed and displayed at 14, as well asrecorded in the memory 11B that may be part of the control unit 14.Then, the load applicator 3 is released, the sensor body 1A is unlockedand then rotated into the second position II, wherein the sensor body islocked again and the load application, sensing and recording is repeatedfor each of the possible positions of the sensor body 1A.

In FIG. 5 the recorded values are shown by the full line curve whichconnects the actually measured values in the measuring positions I toIV. A dashed line curve connects rated values from the memory 11B withina permissible tolerance range TR. As shown, all four measured values atI, II, III, IV are outside the permissible tolerance range TRestablished by the above-mentioned reference values.

First, the output signals in the opposing load directions, namely I andIII, or II and IV are respectively compared with each other in thecomparator 12. If these measured signals or values are equal to eachother in their size, and additionally have opposite plus or minus signs,the evaluation of the output signals can proceed and no further loadapplication cycle needs to be performed. If the size of all outputsignals is within the tolerance range TR, no tuning of the sensor body1A is necessary. The testing can now stop. However, in FIG. 5 all fivemeasured values are outside the tolerance range TR. Therefore, a tuningoperation is required. The fifth measured value is a repetition of thefirst measured value since 0° rotation and 360° rotation of the sensorbody 1A coincide. A mechanical tuning is performed as follows. Thesensor body 1 is rotated into a position in which the output signal hasa positive sign. Next, material is removed mechanically from the sensorbody along the upwardly facing respective ridge, for example at MR shownin FIG. 1. The mechanical material removal must not disturb the straingages 5. The material removal is performed in such a way that smoothtransitions, rather than sharp edges, are provided between any cavitythat results from the material removal MR and the surrounding area ofthe sensor body 1A. According to the measured values shown in FIG. 5,the material removal here takes place along the top edge of the sensorbody 1A in its position I as shown in FIGS. 1 and 2, for example by agrinder with a suitable grinding head shown in FIG. 1 controlled by thetuning device 15.

Upon completion of the material removal MR along the ridge of positionI, the sensor body 1A is brought into rotational position II which alsoyields a positive output signal as shown in FIG. 5. Here again, materialis removed in the manner described above however, in the now upwardlyfacing second ridge. The volume or quantity of material removed dependson the size of the respective output signal from the tuning device 15.FIG. 5 shows that the second signal is positively larger than the firstsignal which means that more material must be removed along the secondridge than along the first ridge.

Upon completion of the material removing steps as described, the sensorbody 1A is now subjected to a new load cycle. It is ascertained whetherall output signals are now within the permitted tolerance range TR. Ifthe newly measured set of values is not within the permissible tolerancerange TR, the mechanical tuning operation by material removal describedabove is repeated by a further material removal or removals until allmeasured values are in the tolerance range TR.

If the tuning resulted in too much material removal, the sign plus orminus of the measured signals will be reversed so that now a tuning inthe opposite direction will have to be made by removing material fromthe respective other ridges at III and IV.

It is a basic fact of strain gage sensors, that the responses of twostrain gages applied to two planes that are at right angles to eachother on a sensor body are independent of each other. As a result, thetuning in the rotational positions I and III or II and IV can beperformed without any mutual influence. In other words, these tuningscan be made independently of each other. However, it is advantageous toperform the loading and measuring sequence completely prior to thematerial removal and to then perform the tuning by material removal,whereby the measuring steps are separated from any machining stepsthereby providing an efficient operation.

Referring further to FIG. 4, the control unit 11 supplies to the input12B of the comparator 12 the above mentioned reference values that arestored in the memory 11B and may differ for different types of pick-ups.The reference values define the tolerance range TR shown by dash-dottedlines in FIGS. 5 and 8. The display unit 14 then shows whether a tuningas described above is necessary or not. If no tuning is necessary, thedisplay unit shows a respective "OK" signal. However, if a tuning isrequired, the tuning device 15 causes a rotation of the sensor body 1Ainto a position that presents the respective ridge of the sensor body 1Ato a material removing tool such as the grinder shown in FIG. 1. Thetuning device 15 controls the operation of the material removing toolshown in FIG. 1 in response to a respective output signal from thecomparator 12 which must coincide with a control signal from the controlunit 11 in accordance with a respective program stored in the memory11B. For this purpose the signal from the comparator 12 first passesthrough the tuning device 15 before it controls the operation of thegrinder shown in FIG. 1. The display 14 may be equipped for indicatingthe angular position in which the material removal takes place and alsoto indicate the quantity of material to be removed and already removed.The material removal tool is merely shown as a grinder. However allconventional tools suitable for such material removal may be used.

FIG. 3 illustrates a modified embodiment of a testing apparatusaccording to the invention. Both, in FIG. 1 and in FIG. 3 the respectiveload applicator 3, 3' applies a bending load to the sensor body 1A, 1'respectively. The chuck 2' mounted in a machine frame MF holds the lowerend LE of the sensor body 1' against rotation. The load applicator 3' isrotatable preferably in 90° steps about the vertical axis of symmetry ASas indicated by the arrow AR. The load applicator 3' has a downwardlyfacing but slanted plane surface 20 in contact with an upwardly facingcambered or three-dimensionally curved surface 21 of the upper end UE ofthe sensor body 1'. The cooperating surfaces 20 and 21 also form a typeof pendulum support. By rotating the load applicator 3' in 90° steps,bending loads are applied in different directions to the sensor body 1'.Otherwise, the sensor body 1' is also constructed with a squarecross-section as described above with reference to FIG. 1. The sensingand measuring of output signals and their evaluation, as well as thematerial removal for the tuning are the same in FIG. 3 as describedabove with reference to FIGS. 1, 2, and 4, as well as 5.

The arrangement shown in FIG. 3 is preferably oriented so that the axisof symmetry AS extends vertically. Four separate material removingtools, such as grinders, may be positioned to cooperate with arespective one of the four edges of the vertically positioned sensorbody 1'. However, a single grinder movable through a range of 360° couldalso be used for the above described material removal along the fouredges of the body 1A or 1'. Although it has been mentioned above thatthe chuck 2' in FIG. 3 holds the body 1' against rotation and theapplicator 3' rotates, the arrangement could be reversed by holding theapplicator 3' in a fixed position and rotating or stepping the chuck 2'in the same manner as has been described for the chuck 2 with referenceto FIG. 1. The effect is the same since it does not matter which of thetwo cooperating surfaces 20 and 21 is stationary and which is rotatingin 90° steps between load applications. No load is applied duringrotation.

Instead of, or in addition to a tuning by means of material removal, itis possible to perform tuning by varying tuning resistances in thecircuit arrangements of the strain gage elements 5. Suitable for thispurpose are, for example, strain gage measuring bridge circuits appliedto the sensor bodies 1A or 1' and having at least four strain gages in abridge circuit, as will be described below with reference to FIGS. 11and 12.

FIGS. 6 and 7 illustrate an arrangement of four strain gages R1, R2, R3,and R4 on the inner bottom surface of a cup-shaped hollow cylindricalsensor body SB having a disk-shaped cover 30 and a cylindrical ring wall32 for mounting the sensor body SB in a machine frame MF. One couldassume that the cover 30 is the bottom of the cup. FIG. 7 is a view intothe open end of the sensor body SB. A load introduction member 31 havinga cambered surface, as described above with reference to FIG. 3,cooperates with a load applicator 3' constructed also as described abovefor the application of the force F. The load introduction member 31 ismounted centrally to the upwardly facing outer surface of the cover 30of the sensor body SB. When a load F is applied while restraining thebody SB against rotation and moving the applicator 3' downwardly, thebody SB, particularly the cover 30, is elastically deformed. Theresulting deformations are sensed by the four strain gages R1, R2, R3and R4. As shown in FIG. 7, the strain gages R1 and R3 are positionedclose to the inner surface of the cylindrical wall 32 of the sensor bodySB while the second pair of strain gages R2 and R4 are positioned closerto the load introduction point, namely closer to the center of the diskcover 30. Thus, the radially outer strain gages R1 and R3 measuredeformations that are downwardly directed in the form of compressions ofthe cover 30 of the sensor body SB, while the strain gages R2 and R4measure tension stress caused by the same load application to the member31 on the cover 30.

Referring to FIG. 11, the four strain gages R1, R2, R3 and R4 areinterconnected to form a bridge circuit having four branches A, B, C,and D. A shunt resistor RI is connected in parallel to the strain gageR3. A series resistor RII is connected in series with the just mentionedparallel connection in the branch D. Bridge input terminals are providedbetween branches AB and CD. Bridge output terminals are provided betweenbranches AC and BD. The shunt resistor RI is adjustable and so is theseries resistor RII. The adjustment of the resistances of the resistorsRI and RII is performed by the operator in response to the outputsignals ascertained during the performance of the present method. As aresult, an electrical tuning is achieved in response to the measuredoutput signal as displayed in the display 14. Such electrical tuning maybe applied either instead or in addition to the above mentionedmechanical tuning by material removal.

In all instances, the output signals are ascertained in response to therespective angular position of the sensor body. For this purpose, thepick-ups of FIGS. 9 and 10 as well as FIGS. 6 and 7 have a zero angularposition 0° as shown in FIGS. 7 and 10. Here again, the angular positioncan be determined either by rotating the sensor body SB and keeping theload applicator stationary or vice versa. It may be preferable to mountthe sensor body in a fixed position in a machine frame MF and rotate theload applicator 3'. However, the present method functions just as wellwhen the applicator 3' is stationary and the sensor body is rotated in90° increments.

The load application in the embodiments 6, 7, as well as 9 and 10 canalso be performed as described above with reference to FIG. 1, whereby aload applicator 3 would engage the load input member 31 to apply a loadhorizontally or vertically, depending on the orientation of the mountingof the sensor body SB or 1A. In FIG. 3, for example, the longitudinalaxis of the load applicator 3' and the longitudinal axis of the sensorbody 1' are oriented vertically, yet the load application direction ofthe effective load component is horizontal due to the cooperation of theslanting surface 20 with the cambered surface 21.

Referring to FIGS. 6 and 7, the operation of this embodiment will now bedescribed with reference to FIG. 8. The zero position 0° of the sensorbody SB corresponds to the off-center load application next to thestrain gage R4. In the angular range between 0° to 180° the loadapplication is coordinated with the bridge branch A formed by the straingage R1 as shown in FIG. 11. In the range between 180° and 360° the loadapplication is coordinated with the bridge branch D formed by the straingage R3. In the range between 90° and 270° the load application iscoordinated with the bridge branch B formed by the strain gage R2. Inthe range of 270° to 90° the load application is coordinated to thebridge branch C formed by the strain gage R4. These coordinations areshown in FIG. 8 by the lines A', B', C' and D', which designate therespective angular sections along the abscissa and which represent therespective bridge branches A, B, C and D.

Upon completion of a load application sequence in the rotationalpositions I, II, III, and IV, that bridge branch is ascertained whichshows to have the highest sensitivity to off-center loads. In theexample of FIG. 8 it is the bridge branch D that has the highestsensitivity in the positive direction. Bridge branch D is formed by thestrain gage R3 which is provided with the above mentioned shunt resistorRI and series resistor RII. The shunt resistor RI provides a damping ofthe respective bridge branch D and the series resistor RII enables therestoration of the bridge symmetry. The resistances RI and RII can beadded to the bridge circuit or the strain gages are provided withrespective resistor branches that can be activated by opening respectiveconductors in the strain gage. In such an arrangement it is preferred toprovide each bridge branch A, B, C, and D with such resistorarrangements forming part of the strain gages. The tuning is thenperformed by opening respective conductors. The sizes of the resistorsRI and RII are determined by experimentation so that the ascertainedvalues are then applicable to all tuning procedures for all pick-ups ofthe same model or type. After the application or activation of theresistors RI and RII, a further series of load applications is performedin order to check whether the output signals provided after thecorrection or tuning are within the tolerance range TR also shown inFIG. 8. It has been found that by varying the resistors RI and RII, itis possible to quickly bring the output signals into the tolerance rangeTR. By providing additional resistances, either for each bridge branchor only for the neighboring bridge branches B or C, activation or tuningof these resistors results in a quick reduction in the off-centersensitivity of the pick-ups to bring the output voltages measured by thestrain gages into the tolerance range TR.

FIGS. 9 and 10 illustrate a pick-up similar to that of FIGS. 6 and 9,however, as also shown in FIG. 12, a total of eight strain gages R1, R2,to R8 are used in this embodiment. Referring specifically to FIG. 10,the strain gages are arranged in pairs, whereby one member of a pair isarranged radially outwardly, while the other member of the same pair isarranged radially inwardly on the inwardly facing surface of the cover30 of the sensor body SB. Strain gages R1 and R5 form a first pair.Strain gages R2 and R6 form a second pair. Strain gages R3 and R7 form athird pair and strain gages R4 and R8 form a fourth pair. Referring toFIG. 12, strain gages R1 and R2 are connected in series with each otherin branch A of the bridge circuit. Strain gages R6 and R7 are connectedin series with each other in branch B of the bridge circuit. Branch D ofthe bridge circuit comprises the strain gages R3 and R4 connected inseries with each other and in parallel with the shunt resistor RI. Thisparallel connection in turn is connected in series pith the resistor RIIin branch D. Strain gages R5 and R8 connected in series with each otherform branch C of the bridge circuit. The illustration of FIG. 8 alsoapplies to the embodiment of FIGS. 9, 10, and 12. Bridge branch D showsthe highest sensitivity to off-center load applications. In theembodiment of FIGS. 9, 10, and 12, adjustments of the resistors RIand/or RII also quickly bring the sensitivity of the pick-up tooff-center load components into the permissible tolerance range TR. Hereit is also possible to provide any of the other bridge branches withtuning or adjustment resistors. The mechanical and/or the electricaltuning may be used in all instances singly or in combination. Materialremoval MR is shown in FIGS. 6 and 9 in the upwardly facing surface ofthe cover 30.

FIG. 13 illustrates the fixed mounting of both sensor body ends andapplying a testing force F centrally between the two mounted ends orpoints. The force F is preferably applied to a longitudinal ridge oredge of the sensor body.

In FIG. 14 the sensor body is still supported at two points or in twoplanes namely P1 and P2 but the testing force F is now applied to a freeend of the sensor body namely outside the two supported points.

FIG. 15 shows the application of a bending moment BM to a sensor bodyrigidly held at one end and introducing two equal but oppositelydirected forces with equal lever arms into the free end of the sensorbody.

FIG. 16 shows the introduction of a torque moment TM into the free endof a sensor body, the other end of which is rigidly held or clamped asshown.

It has been found that a tuning operation for load cells, especially forrocker pin load cells, is also possible if the testing loads are notidentical in each rotational position I, II, III, and IV of the loadcell to be tuned. Rather, the loads applied to the load cell indifferent rotational positions may be different, they may vary,fluctuate or oscillate, provided the circuit that controls the tuningcan compensate for such different loads.

For example, referring to FIGS. 1 and 2, loads F applied in loadapplication positions or points II and IV may be twice as large as theloads applied in points I and III, thus

    2F.sub.I =2F.sub.III =F.sub.II =F.sub.IV.

These different forces applied in different load application points, asthe cell is stepped from point to point, result in different measuredsignals that must be standardized or normalized for providing a propercontrol signal for the tuning operation. For this purpose the measuredsignals are processed to first correlate the measured signals to therespective applied loads F_(I) →S_(I) ; F_(II) →S_(II) ; F_(III)→S_(III) ; and F_(IV) →S_(IV) and then the signals are made uniformeither by amplifying the smaller signals by a known factor or byreducing the larger signals by a known factor. In the assumed examplethe applied forces are as set forth above, whereby the respectivenormalized signals become S'_(II) and S'_(IV) become S_(II/2) andS_(IV/2), respectively.

The normalization or standardization is shown in FIG. 18, wherein themeasured signals S_(I), S_(II), S_(III), and S_(IV) are shown in curveMC based on respective applied load forces 2F_(I) =F_(II) =2F_(III)=F_(IV). Curve S represents the standardized forces F_(I) =F_(II)=F_(III) =F_(IV) or respective standardized signals. The application ofloads twice as large at points II and IV in FIG. 2 as at points I andIII is merely one example. Other loads may be applied at the differentpoints I, II, III, and IV. The loads may be applied off-center, Theloads may be applied to cause bending moments, torsion moments and anyother force applications that cause respective stresses in the sensorbody 1A, 1'. The resulting signals are measured and standardized by arespective signal processing circuit shown in FIG. 17.

FIG. 17 shows a circuit diagram similar to that of FIG. 4, however,modified according to the present invention. The block diagram of FIG.17 shows in addition to the system components of FIG. 4, a force sensoror load cell 20, a memory and signal coordinator 21, and a signalprocessing circuit 22 for tuning load cells with the application ofnon-uniform load forces. The additional components permit the use ofdifferent load application forces applied to different loading points I,II, III, and IV. The term "different load application forces" isintended to include variable loads, oscillating loads, fluctuating loadsand any other type of loads that permits the required tuning. The tuningmakes the load cells less sensitive to off-center load applications.

The force sensor 20 picks up the respective load or force and supplies arespective signal through a conductor 20A to a signal coordinatingcircuit 21 which has also a memory for storing signal information.

The measured signals are coordinated or synchronized with signals fromthe above mentioned angular position sensor 10A which senses for examplethe marker 10 that rotates with the load cell and signifies respectiveload application points. Thus, the signal measured at load point I,shown in FIG. 2, is coordinated to the respective marker signal receivedon conductor 21A from the system control 11 and so forth. This signalsynchronization avoids signal confusion. The signals coordinated totheir respective load application points I, II, III, IV are stored inthe memory of the circuit 21 which also flags the coordinated signalswith signal processing information or signal influencing factorsindicating for example: this signal needs to be reduced or amplified forthe above mentioned standardization. The signal influencing factors may,for example be provided by a comparator or by a threshold circuit.

The signal influencing factors signify, for example, the size of theapplied load in a particular load application point. If, for example,the load F_(II) applied in point II is twice as large as the load F_(I)applied at point II the signal influencing factor for position or pointII will be 0.5, for position or point I it will be 1.0.

It is practical to standardize or normalize the measured signalsrelative to the smallest load F_(MIN) applied at any one of the loadapplication points I, II, III, IV during one 360° load application cycleshown in FIG. 18. This smallest load F_(MIN) also serves for definingthe tolerance range TR shown in FIGS. 5 and 18.

The flagged signals at the output of the circuit 21 are supplied to asignal processor 22 through a conductor 22A. The processor 22 alsoreceives from the amplifier 13 the signals measured by the strain gages5 on the sensor body 1A of the load cell 1 to be tuned or made lesssensitive to off-center load applications. The signal processorstandardizes the signals flagged with an influencing factor as describedabove, for example, the signal S_(II) for the point II is divided by 2to make the standardized signal S'_(II) equal to the signal S_(I) and soforth.

The standardized signals and signals that did not require any change aresupplied to the comparator through a conductor 12A for evaluation asdescribed above with reference to FIG. 4. As shown in FIG. 18, thelarger measured signals shown in curve MC have been standardized asshown in curve S.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated that it is intended to coverall modifications and equivalents within the scope of the appendedclaims.

What we claim is:
 1. A method for making a load cell having a sensorbody with a symmetry axis, less sensitive to off-center loadapplications, comprising the following steps:(a) securing at least onepick-up element to said sensor body and holding the sensor body by atleast one mounting, (b) applying a first load in a first directionrelative to said symmetry axis to said sensor body to produce a firstoutput signal, (c) measuring said first output signal and correlatingsaid first output signal to said first load, (d) applying at least onesecond load of different size compared to said first load to said sensorbody in a second direction different from said first load applicationdirection, (e) measuring a second output signal and correlating saidsecond output signal to said second load, (f) ascertaining from saidfirst and second output signals whether an off-center load sensitivityof said load measuring load cell is within a permissible tolerancerange, and if necessary as a result of said ascertaining (g) performingat least one of a mechanical tuning and an electrical tuning of saidload measuring load cell.
 2. The method of claim 1, comprisingperforming, following a tuning, further load application steps, andascertaining from said further load application steps whether a secondtuning is necessary.
 3. The method of claim 1, comprising performingsaid load application steps in two load application planes whichpreferably extend at right angles to each other.
 4. The method of claim3, comprising performing said load application steps with opposing loadapplication directions in respective load application planes.
 5. Themethod of claim 1, comprising rotating said sensor body about its axisof symmetry, preferably through an angle of 90°, following one loadapplication, and holding said load application device in a fixedposition for each load application.
 6. The method of claim 1, comprisingholding said sensor body at one end in said mounting and applying saidload in an area of the other end of said sensor body.
 7. The method ofclaim 6, comprising providing said load applicator with a plane loadapplication surface (20) slanted relative to said symmetry axis,providing said sensor body (11) with a cambered three-dimensionallycurved end surface (21), and applying a load to said end surface (21) ofsaid sensor body (1') through said slanted surface (21).
 8. The methodof claim 1, comprising holding said sensor body so that said symmetryaxis extends horizontally or vertically.
 9. The method of claim 1,comprising holding said sensor body at two points of said sensor body.10. The method of claim 9, comprising applying a load between said twopoints of said sensor body.
 11. The method of claim 9, comprisingapplying a load outside said two points of said sensor body.
 12. Themethod of claim 1, comprising applying a load by introducing at leastone of a torque moment and a bending moment into said sensor body atleast at one mounting position.
 13. The method of claim 1, comprisingcontrolling rotational steps of said sensor body into load applicationpositions or rotational steps of said sensor body into the tuningpositions in response to a program stored in a memory of a control unit.14. The method of claim 1, comprising performing said tuning by removingmaterial portions (MR) from said sensor body.
 15. The method of claim 1,comprising performing said tuning by adapting at least one of saidpick-up elements applied to said sensor body and a mutual circuitarrangement of said pick-up elements.
 16. The method of claim 15,comprising using at least four pick-up elements, preferably formed asstrain gage strips, interconnecting said pick-up elements to form ameasuring bridge circuit having bridge branches, and tuning at least onebridge branch of the measuring bridge by at least one step of dampingsaid at least one bridge branch by a resistor and modifying of a pick-upelement.
 17. The method of claim 16, wherein said resistor arrangementis formed by a shunting resistor connected in parallel to a bridgebranch and by a series resistor connected in series with the pick-upelement in the respective bridge branch.
 18. An apparatus for making aload cell having a sensor body (1A, SB) with a symmetry axis and atleast one strain gage (5) applied to said sensor body, less sensitive tooff-center load applications, comprising a load application device (3,3') for applying different loads in at least one load application point(I, II, III, IV) of said sensor body, at least one mounting (2, 2') forsaid sensor body, a rotating device (M) for providing relative rotationbetween said sensor body (1A, SB) and said load application device (3,3'), a control unit (11) having at least one input and a number ofcontrol outputs, a rotation sensor (10) connected to said at least oneinput of said control unit (11) to provide an input signal representinga relative rotational fixed position between said sensor body and saidload application device to said control unit (11), a signal amplifier(13) connected to said strain gage (5) to receive output signals fromsaid strain gage (5), a comparator (12) connected to an output of saidamplifier (13) and to a first control output of said control unit (11)providing rated tolerance values for comparing with said strain gageoutput signals, a tuning device (15), said comparator having a firstoutput connected with an input of said tuning device (15), a displayunit (14) connected to a second output of said comparator (12), wherebysaid tuning device (15) is enabled to perform a tuning operation inresponse to said control unit (11) and in response to said comparator,wherein said tuning device (15) has an output connected to a furtherinput (14A) of said display unit (14), said apparatus further comprisingcircuit components (20, 21, 22) for normalizing measured signalsresulting from the application of at least one different testing load inat least one load application point.
 19. The apparatus of claim 18,wherein the load application device (3) comprises a plane slanted loadapplication surface (20) extending at a slant to said axis of symmetryof said sensor body (SB, 1'), and wherein said sensor body (SB, 1')comprises a three-dimensionally curved or cambered end surface (21)cooperating with said plane, slanted load application surface (20). 20.The apparatus of claim 19, wherein said sensor body comprises acup-shaped hollow cylindrical configuration with a flat cover at one endof a cylindrical wall, and wherein said mounting comprises a supportsurface for said cylindrical wall of said sensor body.
 21. The method ofclaim 1, further comprising the step of normalizing measured loadsignals with reference to a load signal selected as a reference loadsignal.
 22. The method of claim 1, wherein said reference load signal isselected to be a minimum load signal resulting from a smallest loadapplied to any one of a plurality of load application points (I, II,III, IV).